In certain parts of an aircraft, for example in the aircraft fuel tank system or in the bleed-air inerting system, it is desirable to have an overpressure protector to relieve excess pressure should it reach a dangerous level.
In a fuel tank system, dangerous pressure differentials may occur when, for example: the aircraft is being refuelled (resulting in high pressure in the tank relative to the atmosphere i.e. positive pressure differential); the aircraft is making an emergency descent (resulting in low pressure in the tank relative to the atmosphere i.e. negative pressure differential); or the fuel venting system between individual tanks becomes blocked (resulting in a large pressure differential across the fuel tanks).
It is known to fit an overpressure protector in the form of an overpressure valve. The overpressure valve is typically in the form of a poppet valve or the like and is adapted to move from a closed position to an open position in the event of an excessive pressure differential. The valve can be reset, typically manually, and does not therefore require replacement after use. The valve tends to be easy to maintain. The valve can also be mounted flush with the outer skin of the aircraft (when used in the surge tank) thereby providing a low-drag arrangement.
It will be appreciated that the above-described overpressure valve thus has a number of advantages. However, a significant problem with the valve is that it can become jammed closed. Since the overpressure valve itself tends to be a back-up pressure release means, if it jams, excess pressure may be allowed to build up and there is a risk of the aircraft structural integrity failing. In, for example, an aircraft fuel system, the overpressure valve may jam due to ice formation in the valve. In such circumstances the primary venting means (for example a flame arrestor/NACA duct) may have also become blocked with ice and there is an especially high risk of excess pressure build-up.
Another known type of overpressure protector is a frangible disc. The frangible disc may be arranged to fail (rupture) when the pressure differential exceeds a certain level. The frangible disc tends to be a simple and light design and is less susceptible to the effects of icing. However, frangible discs cannot be reset and must be entirely replaced after use. This is costly and time consuming. In addition, in an aircraft fuel tank system, the disc tends to be recessed from the wing skin in a tubular housing within the surge tank. This limits spurious failures of the disk but can cause significant drag.